Thermal print head printers are well known and widely used for both single and multicolor applications. Thermal print heads take the form of linear arrays of closely spaced heating elements with each element defining a column of separately controllable printed image pixels. These heating element arrays are held in compressive contact with a heat sensitive print medium directly or through a heat sensitive donor ribbon containing ink, and heat from the elements develops inks within the print medium or transfers ink from the donor ribbon to the print medium. The print density produced by this process is dependent upon various physical aspects, including thermal efficiency of the heating elements, the amount of energy used per pixel, heat transfer characteristics of the heating elements and the heat sink, thermal contact between the heating elements and the thermal medium, etc. Unfortunately, inconsistencies between adjacent elements in any of these variables can result in variations of print density that are visible as streaks on the printed image. This problem is only confounded in higher speed printing applications where thermal characteristics are harder to control due to limited printing time per pixel and an inherent heat build up in the print head between sequentially printed pixels. Aging of the resistive heating elements can also increase the variation in their efficiency and thus print density over time.
It is therefore desirable for the control processes and systems for thermal print head arrays to include aspects for enhancing consistent print density between heating elements of an array to thereby minimize the appearance of image streaks and thus improve image quality.
One such system is described in U.S. Pat. No. 4,827,279, which system calculates a correction value for each heating element after measuring a printed sample on a transparent receiver with a microdensitometer. The respective correction values are then added to image pixel data to be printed by the respective heating elements.